The invention relates to a drilling apparatus including a drilling device that is intended to be fed into a hole to be drilled and which is preferably extendable in the longitudinal direction. The drilling device comprises a casing part essentially inside of which there is at least during a drilling situation a drilling unit in the drilling head of which there are at least a first drilling means for drilling a center hole and a second drilling means for reaming the center hole for the casing part as well as a flushing means for removal of the drilling waste. At least during the drilling situation the rotational movement around the longitudinal axis and the impact movement in the longitudinal direction of the first drilling means is transmitted by a counterpart assembly to the second drilling means that is drivingly connected to the first drilling means essentially at the drilling head of the drilling unit, wherein the second drilling means is arranged to rotate in connection with the head of the casing part centrically around the longitudinal axis by a coupling assembly. The first drilling means is arranged detachable from the second drilling means for removing the first drilling means from the prepared hole, while at least the second drilling means is left in the bottom of the hole.
Patent Publications GB-959955 and GB-1068638 disclose drilling arrangements such as above. The solutions described in both mentioned publications comprise inner drilling means, in other words the center drill for drilling the centerhole and outer drilling means that is symmetrical in relation to the longitudinal axis of the drill and the leaving of which in the hole together with the casing part after the drilling situation is made possible. In such an arrangement, thanks to the centrical rotation movement of the outer drilling means or in other words the reaming drill, the risk of breakage of the drilling arrangement is rather small, especially compared with currently widely used drilling arrangements having eccentric reaming drills.
The contact surface of the reaming drill according to the solution presented in the Patent Publication GB-959955 touches the head of the casing part from the inside. In this case the effective diameter of the center drill is reduced also by twist locking and impact surface assemblies between the center drill and the reaming drill. The mentioned publication presents two differing solutions, wherein a shape locking has been applied as the twist locking assembly in the first solution between the drilling means and in the other one a bayonet coupling between the same. Accordingly, the impact surface assembly comprises a recess-projection assembly between the reaming drill and the center drill that is situated in the front edge of the said twist locking assembly. In a solution described above, the casing part has to be fed into the hole to be drilled by influence of the center drill, wherein the feeding movement is transmitted by means of the counterpart assembly through the reaming drill, in which case the casing part follows the reaming drill. Thus it is practically possible that the impact movement of the center drill is transmitted at least partially also directly to the casing part.
The Patent Publication GB-1068638 discloses a solution in which the reaming drill is placed end to end with the head of the casing part. In this case there is an internal socket fixed in the reaming drill, which is placed in contact with the inner surface of the head of the casing part. In the head of the casing part and in the socket there is a recess-projection assembly, by influence of which the socket remains in place in the longitudinal direction, however allowing rotation of the socket in relation to the casing part. In the solution above there has also been applied an additional block in connection with the arm of the center drill, which couples the rotational movement, feeding movement and impact movement of the center drill to the reaming drill by influence of the socket.
It is common to solutions according to those above that the effective diameter of the center drill is relatively small, that is about 50% of the inner diameter of the casing part. Naturally this is why it is necessary to apply excessively massive drilling rods, which naturally raises the manufacturing costs of the drilling arrangement explained above. Additionally the massiveness of the constructions is also a reason why the handling of the parts of the drilling arrangement is difficult, besides the usage of which demands high capacity. That is why the solutions of above explained types have currently not been used too much in practice, though a centrically rotating reaming drill has many significant advantages compared especially with a so called eccentric reaming drills.